1. Field of the Invention
The present invention relates to battery pack controllers, and more specifically to a battery pack controller that performs control on the voltages and charge/discharge states of unit cells and cell-modules of a battery pack mounted on a motor vehicle.
2. Description of the Related Art
In motor vehicles such as electric cars and hybrid cars, it is the usual practice to assemble a large number of lithium unit cells into a single package, known as battery pack, and bundle their power-feed lines to reduce their wiring cross-section and the loss of their associated switching elements. Since lithium cells are susceptible to over-charging and over-discharging, the voltages developed by the unit cells tend to vary significantly from one cell to another. To eliminate the voltage variability, a cell equalizer is used as an indispensable element to prevent the unit cells from becoming over-charged or over-discharged. One example of such cell equalizer is shown and described in Japanese Patent Publication 2002-325370. Since the number of unit cells in a battery pack is as large as several hundred, they are equally divided into a number of groups, called “modules”, and a plurality of inter-cell cell equalizers are provided, one for each module to equalize the cell voltages of each module. An inter-module cell equalizer may also be used to eliminate a voltage difference that can possibly occur between the modules.
Another indispensable tool for the management of a battery pack involves the use of an overcharge/overdischarge detector that monitors the charge/discharge state of each unit cell and cuts off its charge/discharge currents or provides a preventive measure by controlling the cell currents based on the monitored state. In a hybrid car where a high-voltage battery pack is used, the number of unit cells is significantly large and the voltage difference between the unit cells is high. Because of this reason, the unit cells of the high-voltage battery pack are equally segmented into multiple modules in like manner to the cell equalizer and each module is controlled by a module controller. In addition to the segmentation, the overcharge/overdischarge detector is provided to supply its output to a battery pack controller that provides an overall management of the battery pack in order to achieve simplification of its wiring and isolation of its input and output lines.
Also known is a line monitor that monitors the lines connecting the opposite terminals of each module and the opposite terminals of each unit cell to the module controllers for detecting a possible disconnection, as shown and described in Japanese Patent Publications 2002-325370 and 2003-084015.
However, when the module controllers provide control on the cell equalizer and the overcharge/overdischarge detector in an attempt to reduce cell-to-cell voltage differences while preventing each cell of a module from being overcharged or overdischarged, the operation of the overcharge/overdischarge detector conflicts with the operation of the cell equalizer, causing the voltage of each unit cell of the module to vary significantly. As a result, the charge/discharge state of each unit cell cannot precisely detected. In particular, when the cell equalizer is operating a unit cell in a discharge mode and a disconnection should occur in the line connecting this unit cell to the discharge resister of the cell equalizer, a unit-cell voltage will be impressed in error on a cell charge/discharge monitor that is associated with a unit cell adjacent to the discharge-mode unit cell, resulting in a significant error on the determination of overcharge/overdischarge state of the unit cell.
Furthermore, if line-cut detection is performed when the cell equalizer is operating to equalize cell voltages, the current that flows through the discharge resistor of the equalizer interferes with the line-cut detection and a false line-cut decision could result.